1. Field of the Invention
The present invention relates to a thermosetting resin composition for casting high-voltage coils having good moisture resistance, and to coils and panels formed by casting and curing the resin composition. In more particular, it relates to a thermosetting resin composition for casting high-voltage coils which has a low viscosity and good workability at the time of casting, little of voids and, after cured, develops no cracks when subjected to a heat cycle of from -30.degree. C. to 90.degree. C. and does not decrease its strength and electrical performance characteristic to half even after the lapse of 15 years or more in high humidity atmosphere, and to molded coils and panels formed by casting and curing the resin composition.
2. Description of the Related Art
In recent years, many of the high-voltage coils have come to be operated in high humidity atmospheres such as those in tunnels and underground rooms. In the Yamanashi new test line for magnetic levitation system railway, for example, 80% or more of the ground coils are laid in tunnels of humidities of 90% or more and needed to secure the reliability in mechanical strength and electrical characteristics. The same applies also to transformers installed in tunnels for tunnelling work.
In keeping with the above trend, the insulation technology for such high-voltage coils is making marked progress. In particular, the solventless thermosetting resin casting method has rapidly developed, by virtue of its high electrical reliability, good heat dissipation and excellent moisture resistance, in the field of the manufacture of high-voltage coils used in severe conditions, such as those for direct current motors for vehicles, direct current motors for general industries, transformers, ground coils for magnetic levitation system rail-ways, etc.
Requisites for the aforementioned solventless thermosetting resin composition are as follows. (1) It should have a low viscosity so as to permit easy coil casting. In more particular, it should have a viscosity of 50 poises or less at the casting temperature in the case of vacuum or atmospheric pressure casting and a viscosity of 300 poises or less at the casting temperature in the case of pressure gelation casting. (2) It should have a long pot life. (3) It should no evolve volatile matters so that no void is developed in the course of heat curing. (4) It should exhibit goo electrical and mechanical properties when cured. Solventless thermosetting resin compositions for casting which were in general use as satisfying the above requisites are carboxylic acid anhydride-curing epoxy resin compositions having a low viscosity and filled with fused or crystalline silica having good electrical properties.
High-voltage coils operated in tunnels, underground rooms, etc. are exposed to high humidity
atmosphere of 90% or more humidity over a long period while being subjected to a heavy mechanical or electric load. The cured product of the prior carboxylic acid anhydride-curing epoxy resin composition filled with fused or crystalline silica undergoes rapid deterioration of mechanical and electrical properties when it is in a state wetted by absorption of moisture or water. When a heavy load is mechanically or electrically applied in such a state, the system fails.
Molded coils formed by winding a conductor and casting and curing a thermosetting resin composition for casting high-voltage coils are apt to develop cracks in the insulating layer during the manufacture of the coils or when the coils are subjected to a heat cycle unless the thermal expansion coefficient of the cured product of the resin composition is adjusted to (the thermal expansion coefficient of the conductor .+-.0.2).times.10.sup.-5 deg.sup.-1, as described in Japanese Patent Kokai (Laid Open) Nos. 62-224009 and 2-32508. Therefore, it is necessary for avoiding the development of the cracks to match the thermal expansion coefficient of the insulating layer with that of the conductor, which has a low thermal expansion coefficient. It was found that when the proportion of the filler in the carboxylic acid anhydride-curing epoxy resin composition filled with fused or crystalline silica mentioned above was increased in order to lower the thermal expansion coefficient of the insulating layer, the cured product of the resin composition decreased its strength to half in from 1 to 3 years when exposed to high humidity atmosphere while being subjected to a heavy mechanical or electrical load. Therefore, the development of a resin composition for casting high-voltage coils with high moisture resistance has been eagerly desired.
Careful investigations of the cause of the aforementioned poor moisture resistance of the cured product of the carboxylic acid anhydride-curing epoxy resin composition filled with fused or crystalline silica have revealed the following.
(1) Penetration of water occurs mainly from the interface between the filler and the cured resin. Since a large amount of filler is contained, there is a high possibility of the filler particles striking one another, and the paths of water penetration tend to be readily connected. PA1 (2) Since a large amount of filler is contained, the viscosity of the resin composition at the time of casting operation becomes high, which tends to develop voids. Water tends to stay in the voids and cause the deterioration of the resin composition. PA1 (1) The shrinkage in curing is reduced, resulting in a low stress developed in curing. PA1 (2) The thermal expansion coefficient becomes lower and approaches to that of the coil conductor, so that a lower thermal stress is developed. PA1 (3) The heat conductivity is increased and resultantly the heat dissipation is improved and the temperature distribution becomes more uniform. Consequently, the thermal stress is reduced and the temperature rise is reduced. PA1 (4) Since the filler is less expensive than resin itself, the cost can be reduced. Thus, the addition of a filler lowers the cost, reduces the thermal stress developed at the time of coil manufacture and in the heat cycle, and enhances the reliability of the coil. Therefore, the addition of a filler is indispensable. It is needless to say that the filler itself must have a good moisture resistance. PA1 (1) The viscosity is increased to make casting more difficult. PA1 (2) Development of cracks and permeation of water becomes more apt to occur from the interface of the filler and the resin. PA1 (3) The cured resin becomes more rigid and brittle. PA1 (4) Since the filler is heavier than the base resin, sedimentation of the filler takes place during curing, resulting in nonuniformity of the insulating layer. Therefore, fillers must be used in due consideration of these factors. PA1 .alpha..sub.2 : thermal expansion coefficient of matrix resin PA1 .nu..sub.1 : Poisson's ratio of filler PA1 .nu..sub.2 : Poisson's ratio of matrix resin PA1 .gamma.: correction factor for geometry PA1 E.sub.1 : modulus of elasticity of filler PA1 E.sub.2 : modulus of elasticity of matrix resin. PA1 R(Dp): cumulative percentage by weight from the maximum particle diameter to particle diameter Dp, PA1 Dp: particle diameter, PA1 b and n: constants.
If the proportion of the filler added is reduced, the viscosity at the time of casting is lowered to reduce the void formation and, at the same time, the possibility of the filler particles striking one another becomes low. Thus, the improvement of the moisture resistance is possible. In this case, however, another problem arises in that since the thermal expansion coefficient of the insulating layer becomes high, cracks will develop in the insulating layer at the time of coil manufacture or when the coil is subjected to a heat cycle of from -30.degree. C. to 90.degree. C. At the same time, since the heat conductivity becomes low, the heat distribution in the layer becomes not uniform, which tends to develop thermal stress. Further, the heat generated during electricity application can be dissipated with more difficulty and stays inside the layer to raise the temperature, causing the deterioration of the resin composition.
Further, even if a coil which can endure a heat cycle of from -30.degree. C. to 90.degree. C. can be successfully manufactured, the following problem must be solved. In the case of a driving coil for a linear motor car, for example, previously the whole of the coil manufactured was firmly fixed with bolts, spacers and the like or embedded directly into concrete to oppose the electromagnetic force. In such cases, cracks were apt to develop in the coil or concrete owing to the thermal stress caused at the time of heat cycle of -30.degree. C. to 90.degree. C. Further, the fixed parts tended to loosen or undergo creep fracture. Thus, the system was lacking in reliability.
In view of the aforementioned situations, the object of the present invention is to provide a thermosetting resin composition for casting high-voltage coils of good moisture resistance which has a low viscosity and good workability at the time of casting operation and, after curing, gives little of voids in the cured product, develops no cracks even when subjected to a heat cycle of from -30.degree. C. to 90.degree. C. and does not decrease its strength and electric performance characteristic to half even after exposed to high humidity atmosphere of 90% or more humidity over 15 years or more while being subjected to a heavy mechanical or electrical load, and also molded coils and panels formed by casting and curing the resin composition.